1. Field of the Invention
The present invention relates to an image forming method for developing a latent image such as an electrostatically charged image, and an image forming apparatus.
2. Related Background Art
As dry developing in electrophotography, a process employing a one-component magnetic toner as a developer and a process employing a two-component developer comprised of a carrier and a toner are commonly in wide use. The two developing processes making use of the developer have respectively the following advantages, i.e., the advantages that in the one-component developing system the developing apparatus can be made small-sized and in the the two-component developing system the design tolerance can be set in a wide range because of its capability of sufficiently imparting electric charges to the toner.
The above advantages, however, are directed to compensation of problems involved in each other's system. As will be seen from this fact, the one-component system has the problem of a narrow range of design tolerance for the toner and developing system because of its tendency to an unsatisfactory process of imparting electric charges to the toner, and on the other hand the two-component system has the problem of a complicated construction of the apparatus because the toner and the carrier must be mixed in a concentration controlled to a constant value.
With regard to copied images obtained by the respective systems, in the case of the magnetic one-component system the development is carried out while the toner is formed into chains (commonly called "ears") under application of a magnetic field at the time of development, and hence the resolution of an image in the lateral direction tends to become poor compared with that in the longitudinal direction. For example, the phenomenon called "smeared image trailing edge" tends to occur which is due to the protrusion of ears to a non-image area of the latter half of a developed image, and also coarse images tend to occur compared with the two-component system. On the other hand, in the two-component system, a brush image trail of a magnetic brush tends to occur.
As stated above, a disadvantage of the magnetic one-component system is for one thing that the process of imparting electric charges tends to become unsatisfactory, and that the magnetic toner contains a magnetic material which may cause a lowering of image reproducibility.
As a method for settling the problem on image reproducibility, it can be considered effective to make the ears of a magnetic toner shorter and denser. As a means therefor, it can be readily expected to decrease the proportion of the amount of a magnetic material in the magnetic toner. Since, however, the existing developing system commonly depends on an absolute value of specific charges of magnetic toners, the magnetic toner in which the amount of a magnetic toner contained has been decreased tends to make it difficult to carry out development, for the following reasons.
When the radius of one particle of the magnetic toner is represented by R, the charge quantity by Q and the density by .rho., the specific charge Q/M of the magnetic toner is expressed as follows: EQU Q/M=3Q/4.pi..rho.R.sup.3
The density of a magnetic material in toner of commonly available magnetic toners is several times larger in its value than the density of a binder resin. Hence, the density .rho. of the magnetic toner decreases with a decrease in the amount of a magnetic material in toner and the specific charge increases inversely. The increase in the specific charge tends to cause a decrease in image density. In particular, as is clear from the above expression, this tendency becomes more remarkable as toner particles are made to have a smaller particle diameter.
Image quality of copied images in the one-component magnetic developing system is greatly influenced by aggregation properties of magnetic toners, and faulty images such as fogging tend to occur as the aggregation properties become stronger. This phenomenon of fogging is understood to be due to ears each having become thick when magnetic toners have strong aggregation properties, which consequently make it difficult for magnetic toner particles present inside ears to be statically charged by friction with a developer carrying member, resulting in an insufficient electric charge of magnetic toner particles.
The relationship between the intensity of magnetization of magnetic toners and the shape of each ear is also understood qualitatively as follows: When the intensity of magnetizaiton of a magnetic toner is great, a strong attraction force in the direction of the magnetic field and a strong repulsion force in the direction perpendicular to the magnetic field act between magnetic toner particles. Hence, when the intensity of magnetizaiton is great, the ears formed by the magnetic toner become long and coarse and also each ear becomes slender. Inversely, when the intensity of magnetization of a magnetic toner is small, the ears become short and dense in turn and also each ear becomes thick and short because of no loosening of the combination between magnetic toner particles, resulting in an aggregated state. Hence, in the latter case, the magnetic toner particles present inside the ears tend to be insufficiently statically charged as previously stated.
Thus, an attempt to merely decrease the proportion of the amount of a magnetic material in the magnetic toner to make the ears shorter has brought about the problem that charge-up of the magnetic toners or deterioration of image quality such as fogging accompanied with faulty charging of the magnetic toner tends to occur.
In addition, since the magnetic material also serves as a colorant, merely decreasing the amount of the magnetic material may result in an insufficiency of image density.
As a method for making the ears of magnetic toners shorter and dense, one may contemplate the construction in which a developer layer thickness control member comprised of a resilient material is brought into contact with the developer carrying member. In such a construction, however, the developer layer thickness control member comprised of a resilient material tends to be abraded, bringing about the problem of a poor running stability.
Thus, as discussed above, it is earnestly sought to provide an image forming method, and an image forming apparatus, that can control the layer thickness of the developer on a developer carrying member in the state of non-contact with the developer carrying member, can make the intensity of magnetization of a magnetic toner smaller without decreasing the amount of a magnetic material in the magnetic toner, and can improve charging stability and image reproducibility.